This invention relates to cardiac pacers and relates more particularly to pacers having enhanced electromagnetic interference (EMI) protection properties.
Programmed pacers may be functionally classified as either demand or synchronous devices. In the demand pacer, the generation of artificial cardiac pulses is inhibited during normal heart function, while in the synchronous pacer, artificial stimulating pulses synchronized with normal heart activity are generated. In the absence of cardiac activity, both types of pacers switch to a fixed rate mode and supply cardiac stimulating pulses at a predetermined fixed rate, typically in the order of about 70-80 pulses per minute.
Of the two basic programmed pacer types, the effect of electromagnetic interference is more severe in the demand pacer. This is because demand pacers are designed to deliver pacing pulses only when the natural heart rate falls below a predetermined minimum rate. This type of unit is therefore designed to shut off whenever the sensed heart rate (or any other detected input signal) is above the predetermined minimum rate. Accordingly, whenever an interference signal above this rate is detected, the demand pacer will be completely inhibited. If this phenomenon occurs during a period of cardiac malfunction that requires artificial pacing, the results may be fatal.
The consequences of electromagnetic interference detection in the synchronous configuration are somewhat less severe, and a synchronous pacer having enhanced performance in interference fields is disclosed in our prior U.S. Pat. No. 3,949,759.
The dangers associated with electromagnetic interference detection in demand pacers have been well documented, and although various partial solutions have been proposed, the performance of prior art demand pacers in certain types of interference fields has heretofore been unsatisfactory. Nevertheless, the demand pacer is preferred in many applications because it offers a substantial advantage over the synchronous pacer in the area of power consumption. Since the demand pacer does not generate artificial heart stimulating pulses when the patient's natural cardiac pacing and conduction systems are functioning properly (a condition which may exist in some patients for over 90% of the time), the use of a demand pacer rather than a synchronous device may result in substantially reduced battery drain.
Because of this substantial energy consumption advantage, considerable effort has been directed to overcoming the principal drawback of the demand pacer, namely its lack of safety in EMI fields. A typical prior art approach to the problem is shown in U.S. Pat. No. 3,877,438, in which a rate-sensitive interference rejection circuit is provided in a demand pacer. The function of this interference circuit, which is placed between the QRS detector and the pulse generator, is to prevent detected pulses from inhibiting the fixed-rate pulse generator if such pulses occur at too fast a rate. The detection of pulses at above a predetermined rate, which must necessarily be above the range of normal cardiac activity rates, is deemed to be an indication of detection of some kind of "noise", such as 60 Hz pickup, and the pacer is switched to its fixed-rate mode for so long as such noise is detected. The principal drawback of this type of prior art interference rejection scheme is that it is inherently ineffective in the presence of EMI fields which mimic or mock normal cardiac repetition rates. Numerous well-documented interference sources, including ignition systems, household appliances, power tools and medical support equipment, as well as an ever-growing list of new interference sources, such as microwave ovens with stirring devices and anti-theft devices, can all produce pulsed EMI outputs which may mimic the rates of normal heart activity. This mimic condition is particularly severe if the pulsed EMI amplitude is high enough to be demodulated by input defibrillator protection circuits or by amplifier saturation. Such interference sources may thus inhibit prior art demand pacers, in spite of the inclusion of conventional rate discrimination interference rejection circuits, with possibly fatal results. Furthermore, conventional filtering and shielding techniques have proven ineffective, since the amplitude of detected EMI may exceed normal cardiac signal levels by a ratio of over 10,000 to 1.
Various prior art systems and circuits for reducing the effect of EMI on demand pacers are shown in U.S. Pat. Nos. 3,866,616, 3,911,929, 3,926,197 and 3,927,677. However, none of the interference rejection schemes disclosed therein are capable of rejecting interference signals with repetition rates which closely mimic natural cardiac activity.